[PARISC] clocksource: Move update_cr16_clocksource later in boot

smp_cpus_done is too early for us... before we even do a device
inventory! Move update_cr16_clocksource into the tail end of
processor_probe() and stub it out on CONFIG_SMP=n builds.

Verified that clocksource0 is properly updated to use jiffies
on an SMP build.

Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>